Motion detection in medical systems

ABSTRACT

In a medical system, at least one medically operative member ( 10, 12, 100 ) is configured to interact with or acquire data from a subject ( 74 ) disposed in an examination region. An array of photosensors ( 70, 170 ) is disposed on the at least one medically operative member. The array of photosensors is arranged to view the examination region. A position-determining member ( 82, 82   a,    82   b ) is configured to determine a position of at least one optically detectable marker ( 72, 172 ) disposed with the subject in the examination region based on light from the at least one optically detectable marker sensed by the array of photosensors.

The following relates to the medical imaging, radiation therapy, andrelated arts. In some embodiments it relates to gamma camera systems,and is described with illustrative reference thereto. However, thefollowing will find application in substantially any type of medicalimaging system such as gamma cameras, positron emission tomography (PET)systems, magnetic resonance (MR) systems, or so forth in order toprovide dynamic information regarding subject position or motion. Thefollowing will further find application in substantially any type ofmedical therapy system, such as radiation therapy systems, in whichaccurate dynamic knowledge of subject position or motion isadvantageous.

It is advantageous for a patient to remain stationary during certainmedical procedures, such as medical imaging, radiation therapy, and soforth. In practice, however, the patient may move, and in some casesmust move, during the procedure. For example, the patient must breathduring any procedure that takes more than a minute or so to perform. Forlonger procedures, there is a likelihood that the patient may shift hisor her position at some point during the procedure due to fatigue,nervousness, or so forth. The likelihood of patient movement generallyincreases with increased duration of the procedure, and is also enhancedin the case of children and frail adults. Patient motion can be reduced,but not eliminated, through the use of patient restraints. However, mostpatients do not like to be placed into such restraints.

Recognizing a possibility of patient movement, one remedial approach isto compensate for the patient movement. Doing so entails determiningpatient position during the medical procedure. Some known approachesindirectly determine patient position, for example by using pressuretransducers disposed on or in a belt around the patient to detectrespiration. These indirect approaches generally entail some sort ofmodeling or other data transformation in order to extract a patientposition estimate.

Another approach is to use cameras to monitor the patient during theprocedure. For example, R. D. Beach et al., “Feasibility ofstereo-infrared tracking to monitor patient motion during cardiac SPECTimaging”, IEEE Trans. Nucl. Sci. vol. 51, pp. 2693-2698 (2004) disclosesa method employing a plurality of cameras that monitor the patient.Reflective markers are disposed on the patient, and images acquired bythe cameras are analyzed by stereoscopic image processing techniques todetermine the positions of the reflective markers. This approach has theadvantage of being a more direct measurement of patient position, andthe reflecting spheres can be made small and compact and are generallynot discomforting to the patient.

However, using cameras to monitor the patient is not feasible if thepatient, or the portion of the patient which is of interest, is occludedfrom view. A gamma camera, for example, is typically operated with itsdetector heads located conformally as close as practicable to thepatient during imaging, thus substantially occluding from view theportion of the patient that is of interest. Similarly, the imagedportion of a patient disposed in the bore of a magnetic resonance (MR)scanner or positron emission tomography (PET) scanner is substantiallyoccluded from view by the scanner. The stereoscopic camera system alsooccupies a substantial amount of valuable space, and in interventionalprocedures medical personnel must take care not to block the cameras'view of the patient.

The following provides improvements, which overcome the above-referencedproblems and others.

In some embodiments disclosed herein as illustrative examples, aposition measurement system is disclosed, comprising: an array ofphotosensors configured to be disposed on a medically operative memberconfigured to interact with or acquire data from a subject disposed inan examination region, the array of photosensors arranged to view theexamination region; and a position-determining member configured todetermine a position of an optically detectable marker disposed with thesubject in the examination region based on light from the opticallydetectable marker sensed by the array of photosensors.

In some embodiments disclosed herein as illustrative examples, a medicalsystem is disclosed, comprising: at least one medically operative memberconfigured to interact with or acquire data from a subject disposed inan examination region; an array of photosensors disposed on the at leastone medically operative member, the array of photosensors arranged toview the examination region; and a position-determining memberconfigured to determine a position of at least one optically detectablemarker disposed with the subject in the examination region based onlight from the at least one optically detectable marker sensed by thearray of photosensors.

In some embodiments disclosed herein as illustrative examples, aposition measurement method is disclosed, comprising: detecting at leastone optically detectable marker disposed on a subject in an examinationregion using photosensors disposed on a medically operative memberconfigured to interact with or acquire data from the subject in theexamination region; and determining at least one optical marker positioncorresponding to the at least one optically detectable marker based onthe detecting.

One advantage resides in providing a position-determining system fordetermining patient position during a medical procedure.

Another advantage resides in providing a position-determining system fordetermining patient position during a medical procedure in which amedically operative member substantially occludes from view at least arelevant portion of the patient.

Another advantage resides in providing a position-determining system fordetermining patient motion during a medical procedure.

Still further advantages of the present invention will be appreciated tothose of ordinary skill in the art upon reading and understand thefollowing detailed description.

The drawings are only for purposes of illustrating the preferredembodiments, and are not to be construed as limiting the invention.

FIG. 1 diagrammatically shows a perspective view of a gamma camerasystem including a sub-system for determining patient position andmovement.

FIG. 2 diagrammatically shows a sectional view of one of the detectorheads of the gamma camera system of FIG. 1.

FIG. 3 diagrammatically shows a sectional view of the gamma camerasystem of FIG. 1 in which the position-determining sub-system includesphotosensors with wide viewing angle, and the position-determiningmember includes Anger-type logic electronics.

FIG. 4 diagrammatically shows a sectional view of the gamma camerasystem of FIG. 1 in which the position-determining sub-system includesphotosensors with narrow viewing angle so as to define opticalprojection data, and the position-determining member includingback-projection electronics.

FIG. 5 diagrammatically shows a sectional view of the gamma camerasystem of FIG. 1 in which the position-determining sub-system includesphotosensors disposed behind the detector head collimator so as todefine optical projection data, and the position-determining memberincluding back-projection electronics.

FIG. 6 diagrammatically shows a sectional view of a magnetic resonancescanning system including a sub-system for determining patient positionand movement.

With reference to FIG. 1, an illustrated gamma camera system includestwo radiation detector heads 10, 12. In other embodiments, the gammacamera may include one, two, three, four, five, six, seven, or moredetector heads. The radiation detector heads 10, 12 have respectiveradiation-sensitive faces 14, 16, which are generally arranged duringoperation to face a patient support or pallet 18. The illustrateddetector heads 10, 12 are supported by respective articulated,multi-jointed robotic arms 20, 22 that each include a combination ofelectronically controllable translational, rotational, swivel, or othermechanical joints that cooperatively or collectively enable severaldegrees of movement freedom, such as radial movement of the detectorheads 10, 12 toward or away from the patient couch 18, tangentialmovement of the heads in a direction transverse to the radial movement,circumferential movement, or so forth. Camera electronics 24 power andcontrol the articulated robotic arms 20, 22 and the detector heads 10,12, and acquire radiation detection information from the detector heads10, 12. The camera electronics 24 are optionally coupled with a videomonitor 26 for displaying information about the status and operation ofthe gamma camera. The camera electronics 24 include a robotic controller28 that is operable by a hand controller 30 in a manual mode tomanipulate the detector heads 10, 12 using the robotic arms 20, 22, andis operable by suitable control algorithms implemented by the controller28 to move one or both detector heads 10, 12 along predetermined pathsor trajectories. For example, in cardiac imaging the two detector heads10, 12 are typically positioned 90° apart around the subject support 18.In some embodiments, the illustrated gamma camera including theradiation detectors 10, 12, patient support 18, robotic arms 20, 22,camera electronics 24, and video display 26 is suitably embodied by theSkylight™ nuclear camera (available from Philips Medical Systems,Eindhoven, The Netherlands), although other gamma cameras can be used.

With continuing reference to FIG. 1 and with further reference to FIG.2, each detector head 10, 12 includes a collimator 42 mounted on theradiation-sensitive face 14, 16. The collimator 42 is in someembodiments a pinhole, honeycomb, or other type of collimator made ofradiation absorbing material and having pinholes, openings, or filledradiation transmissive regions that collimate incoming radiation in adirection generally transverse to the generally planarradiation-sensitive face 14, 16. In some embodiments, the collimator maybe omitted, for example if the detector head is being used for positronemission tomography (PET) imaging. The illustrated detector heads 10, 12each further include an array of radiation detectors 44. The illustratedradiation detectors 44 include one or more scintillators 46 opticallycoupled with a plurality of photomultiplier tubes 48 arranged to viewthe one or more scintillators 46. The photomultiplier tubes 48 areoptionally replaced by another type of light sensor, such as an array ofphotodiodes. The term “light” as used herein encompasses visible lightand also invisible ultraviolet or infrared light. The outputs of thephotomultiplier tubes 48 are input to electronics, for example disposedon a printed circuit board 50. Instead of the scintillator-basedradiation detectors 44, a plurality of radiation sensitive elements,such as, for example, solid state cadmium-zinc-telluride (CZT) baseddetectors, can be used to directly absorb radiation and output anelectrical signal responsive thereto.

To acquire tomographic imaging data, the robotic controller 28manipulates the detector heads 10, 12 using the robotic arms 20, 22 tomove the detector heads 10, 12 around a subject disposed on the patientsupport or pallet 18. In planar imaging, the detector heads 10, 12 arenot moved around the subject, but may optionally be moved linearly toscan a desired field of view. The detector heads 10, 12 collecttomographic, planar, or other imaging data that are stored in an imagingdata memory 60. For example, the imaging data may be projection data inthe case of SPECT imaging, or line-of-response data if the gamma camerais being used in a PET imaging mode, or so forth. A reconstructionmember 62 applies a suitable reconstruction algorithm, such as afiltered backprojection reconstruction algorithm, an iterativereconstruction algorithm, or so forth, to compute a reconstructed imagefrom the collected imaging data. The reconstruction member 62 istypically embodied as electronics such as a processor or controllerexecuting a reconstruction algorithm; however, the reconstruction member62 may be otherwise embodied, for example as an analog, digital, ormixed application-specific integrated circuit (ASIC) electronicsoperating alone or in conjunction with a controller or processor. Thereconstructed image is stored in an image memory 64, and may bedisplayed on the display of a user interface 66, or stored for laterretrieval in an electronic, magnetic, or optical memory, or transmittedvia a local area network or the Internet, or processed bypost-reconstruction image processing, or otherwise utilized. In theillustrated embodiment, the user interface 66 also provides userinterfacing with the camera electronics 24. In other embodiments, thevideo monitor 26, hand controller 30, or another user interfacing devicemay be used instead of or in addition to the user interface 66 toprovide user interfacing with the camera electronics 24.

The gamma camera described with reference to FIGS. 1 and 2 is anillustrative example. More generally, the position sensing apparatusesand methods disclosed herein can be practiced with substantially anytype of gamma camera. For example, it can be used in conjunction withgamma cameras having a larger number of smaller detector heads. In someembodiments, the robotic arms 20, 22 are replaced by a ring gantry 20′(drawn in phantom in FIG. 1) that supports the detector heads 10, 12. Inthese embodiments, the ring gantry 20′ includes a rotatable gantryportion supporting the heads 10, 12 so as to enable revolving of theheads 10, 12 around the couch 18, and gamma detector head mountingfixtures (not shown) provide for radial and tangential movement of thedetector heads. Still further, the position sensing apparatuses andmethods disclosed herein can be practiced with other medically operativemembers such as a magnetic resonance scanner, a positron emissiontomography (PET) scanner including a ring of detectors, a radiotherapysystem, or so forth.

Having described the gamma camera illustrated in FIGS. 1 and 2 as anexample application, illustrative position-determining systemembodiments are now described with reference to FIGS. 1 and 2 and withfurther reference to FIGS. 3-5. The illustrated position-determiningsystems make use of optically detectable markers disposed on the subjectand photosensor arrays disposed on the detector heads 10, 12. Thisconfiguration advantageously places the photosensor arrays close to thesubject during the imaging, and as will be disclosed herein accurateposition determination is readily obtained based on light sensed by aplurality of photosensors. Moreover, since the photosensor arrays aremounted on the detector heads 10, 12, the same spatialframe-of-reference is employed for both the gamma camera imaging and theposition determining system, and accordingly relative positionalmisalignment between the radiation detector heads and the photosensorarrays is not an issue.

With continuing reference to FIGS. 1 and 2 and with further reference toFIG. 3, each detector head 10, 12 further includes a generally planararray of photosensors 70 disposed on its respective radiation-sensitiveface 14, 16. One or more optically detectable markers 72 are disposedwith a subject 74 placed in the examination region of the gamma camerafor imaging. The optically detectable markers 72 can be light emittingdiodes (LEDs), reflective markers or phosphorescent markers operating inconjunction with ambient or externally applied light, or so forth, andare disposed with the subject, that is, on the subject directly orindirectly. For example, the optically detectable markers 72 may bedisposed indirectly on the subject by being disposed onelectrocardiographic leads attached to the subject, or on clothing wornby the subject, or otherwise disposed with the subject. In someembodiments, it is contemplated for the optically detectable markers tobe sewn into or otherwise attached with clothing that is worn by thesubject. For accurate position determination, it is advantageous for theclothing in such a case to be relatively tight-fitting, that is, tightenough to avoid sagging. The generally planar array of photosensors 70can be pixelated large-area detectors manufactured using amorphoussilicon technology, large-area charge coupled device (CCD) arrays, or soforth.

Some suitable amorphous silicon-based photosensor arrays are described,for example, in Schiebel et al., “Fluoroscopic x-ray imaging withamorphous silicon thin-film arrays”, SPIE vol. 2163 Physics of MedicalImaging (1994) pp. 129-140. The arrays described in Schiebel et al. arecoated with a CsI:Tl scintillation layer and an optional white powderreflective light trapping layer to make them sensitive to x-rays. Forthe present application, the CsI:Tl and white powder layers are suitablyomitted, and the optically detectable markers 72 and the array ofphotosensors 70 are selected for compatibility, that is, the array ofphotosensors 70 should be sensitive to light emanating from theoptically detectable markers 72. The array of photosensors is optionallycoated with a phosphor layer, wavelength-selective reflecting orabsorbing layer, or other layer configured to enhance sensitivity tolight emitted by, reflected by, or otherwise emanating from theoptically detectable markers 72. As noted previously, the term “light”as used in this application encompasses both visible light and invisibleinfrared or ultraviolet light. With the CsI:Tl layer omitted, thepixelated amorphous silicon photosensor array layer is substantiallytransmissive for 511 keV gamma rays detected in PET imaging and forradiation emitted by typical radiopharmaceuticals used in SPECT imaging.Accordingly, placement of the thin array of photosensors 70 between theexamination region and the array of radiation detectors 44 is notproblematic. The amorphous silicon thin-film photosensor arrays ofSchiebel et al. are suitably used on thin plastic sheets, glass, oranother base material.

With reference to FIG. 1, for determining the positions of one or moreoptically detectable markers 72 a spatial resolution on the order ofbetween about 0.2 millimeters and 20 millimeters is typicallysufficient, although higher or lower resolutions are also contemplateddepending upon the application. Optical data collected by the arrays ofphotosensors 70 on the radiation-sensitive faces 14, 16 of the detectorheads 10, 12 are temporarily stored in an optical data buffer 80, andare processed by a position-locating member 82 to generate opticalmarker positions 84 for the one or more optically detectable markers 72.Typically, the position-locating member 82 is embodied as electronicssuch as a processor or controller executing a position-determiningalgorithm. However, the position-locating member 82 may be otherwiseembodied, for example as an analog, digital, or mixed ASIC electronicsoperating alone or in conjunction with a controller or processor. Thegenerated optical marker positions 84 are suitably used by the imagereconstruction member 62 to adjust at least one of the collected dataand the reconstructed image based on subject position inferred from theoptical marker positions 84. For example, reconstructed images acquiredat different times can be spatially registered by translating,stretching, shrinking, or otherwise transforming the images such thatthe marker positions are aligned. Alternatively, the imaging data andreconstructed images may be left unadjusted, but subsequent diagnosticinformation derived from the images may be adjusted based on patientmotion known from the generated optical marker positions 84, or certainimaging data or reconstructed images may be discarded based onproblematic patient motion identified based on the generated opticalmarker positions 84.

In the embodiment of FIG. 3, the photosensors of the array ofphotosensors 70 are disposed “on top of” the collimator 42. Thephotosensors of the array of photosensors 70 in the embodiment of FIG. 3have viewing angles effective for a plurality of substantiallycontiguous photosensors of the array of photosensors 70 tosimultaneously sense light from a given optically detectable marker. Forexample, in FIG. 3 a contiguous group CG of photosensors of the array ofphotosensors 70 detect light L from a nearby one of the opticallydetectable markers 72. In the embodiment of FIG. 3, theposition-determining member 82 includes Anger-type logic electronics 82a that use a centroiding or “Anger-type” logic to determine the positionof the nearby optically detectable marker based on relative sensed lightintensities of the photosensors of the contiguous group of photosensorsCG that sense light from the nearby optically detectable marker.Conventional Anger logic is typically used in conjunction with thearrays of photomultiplier tubes 48 of a detector head 10, 12 (see FIG.2) to localize a radiation event on the radiation-sensitive face 14, 16of the detector head.

For the optical application of FIG. 3, however, it is additionallydesired to estimate the distance between the nearby one of the opticallydetectable markers 72 and the centroid of detection on the array ofphotosensors 70. (Mathematically, this amounts to estimating thesmallest distance between a point in space corresponding to theoptically detectable marker and a plane corresponding to the array ofphotosensors 70). The Anger-type logic electronics 82 a suitablyestimate this distance based on the absolute sensed light intensities(higher intensity corresponding to a closer marker) and/or based on thesize of the contiguous group of sensing photosensors CG (larger groupsize corresponding to a more distant marker). The former approach iscomputationally straightforward, and can be implemented for example as alook-up table relating absolute intensity (integrated over thecontiguous group CG of sensing photosensors, or alternatively theabsolute intensity at the centroid of the detecting contiguous group CG)with distance. The latter approach advantageously is not based onabsolute intensity value, and can provide an accurate distance estimatebased on a known relative sensitivity-versus-angle profile for thephotosensors. For example, assuming an abrupt cutoff angle for thephotosensors of “θ” measured off of the array normal, if the contiguousgroup CG is circular with a radius of “x” then the distance “d” can beestimated from the relationship tan(θ)=d/x.

With reference to FIG. 4, in another embodiment the viewing angle of thephotosensors of the array of photosensors 70 is limited or collimated bythe use of an optical collimator 90. In some embodiments, the opticalcollimator 90 is embodied as a pair of crossed micro-laminated opticalfilters such as Vikuiti™ films (available from 3M Company, St. Paul,Minn.). However, in general any planar collimator that is substantiallytransmissive for radiation detected by the detector heads 10, 12 issuitable.

Because of the collimation, light sensed by the generally planar arrayof photosensors 70 defines optical projection data (indicated in FIG. 4by dashed lines). If two non-parallel planar arrays of photosensors 70on different detector heads detect the same one of the opticallydetectable markers 72, then the optically detectable marker is suitablylocated at the intersection of the optical projections. Thus, theposition-determining member 82 can determine the position of theoptically detectable marker based on an intersection of opticalprojections acquired by the two arrays of photosensors 70. However, asseen in FIG. 4 complexity can arise since it is possible for projectionsto intersect at places where there is no optically detectable marker.(It should be noted that the two-dimensional rendering of FIG. 4 makessuch inadvertent intersections appear much more likely than theyactually are in three-dimensional space, where inadvertent intersectionswill be relatively rare for a small number of optically detectablemarkers 72).

In some embodiments, this complexity is addressed by theposition-determining member 82 being embodied as back-projectionelectronics 82 b that determine the position of the optically detectablemarkers based on intersections of optical projections by using knownbackprojection algorithms (or variants such as filtered backprojection,iterative backprojection, or so forth) such as are optionally employedby the reconstruction member 62 to reconstruct SPECT projection data. Inanother approach, inadvertent intersections are detected based oncontinuity—as an optically detectable marker moves the correspondingintersection will track it continuously in space and time, whereas aninadvertent intersection of projections will generally be an abruptevent that will disappear once either of the markers generating theinadvertent intersection move.

With reference to FIG. 5, in another embodiment the generally planararray of photosensors 70 of each detector head 10, 12 is disposed“behind” the collimator 42 and in “front” of the array of radiationdetectors 44 (see FIG. 2). In other words, the collimator 42 ispositioned between the generally planar array of photosensors 70 and theexamination region, and the generally planar array of photosensors 70 ispositioned between the array of radiation detectors 44 and thecollimator 42. In this way, the collimator 42 provides collimation forboth radiation of interest (that is, radiation emitted from the subjectand detected by the detector heads 10, 12 to generate imaging data) andlight from the one or more optically detectable markers 72 such thatradiation of interest detected by the radiation-sensitive face 14, 16defines projection data and light sensed by the generally planar arrayof photosensors 70 defines optical projection data. Theposition-determining member 82 is suitably the same as in the embodimentof FIG. 4, such as for example the illustrated backprojectionelectronics 82 b configured to reconstruct the optical projection datato determine the position of the one or more optically detectablemarkers 72 disposed with the subject 74. In the embodiment of FIG. 5, ifa cosmetic cover (not shown) is disposed over the collimator 42, then itshould be substantially transparent for light emanating from theoptically detectable markers 72.

In the embodiments of FIGS. 3 and 4, an issue can arise if thecollimator 42 is removable. In some gamma camera systems, thecollimators 42 of the detector heads 10, 12 are removable to enableswapping of collimators for different applications. For example, acollimator with relatively larger openings may provide a higher numberof radiation counts (effectively translating to higher signal level)) atthe expense of reduced collimation and concomitant loss of resolution. Acollimator with relatively smaller openings, on the other hand, providesgreater collimation and higher resolution, but may reduce the number ofreceived radiation counts. In the embodiments of FIGS. 3 and 4, thegenerally planar array of photosensors 70 is mounted on the collimator42—accordingly, if the collimator 42 is removable then there ispreferably a detachable electrical connector (not shown) provided toselectively electrically connect the generally planar array ofphotosensors 70 with the printed circuit board 50 or other detector headelectronics. For example, mating connector parts can be disposed on thecollimator 42 and on the detector head such that when the collimator 42is mounted to the detector head the electrical connection isautomatically effectuated. In other embodiments, a manual electricalconnection is provided, and the radiologist or other operator intervenesto electrically connect the generally planar array of photosensors 70.

With reference to FIGS. 1-5, illustrative embodiments have beendescribed in which the medically operative member is a gamma camera andthe array or arrays of photosensors 70 is or are disposed on aradiation-sensitive face or faces of the detector head or heads. Moregenerally, the disclosed position-determining systems are useful inconjunction with substantially any type of medically operative memberthat substantially occludes from view at least the relevant portion ofthe subject with which the at least one medically operative member isconfigured to interact or from which the at least one medicallyoperative member is configured to acquire data. The disclosedposition-determining systems are also useful in conjunction withsubstantially any type of medically operative member that is configuredto move respective to the subject during operation of the medicallyoperative member. In such latter cases, the array of photosensors can beadvantageously be disposed on the at least one medically operativemember so as to move in fixed relative spatial relationship with the atleast one medically operative member during operation of the medicallyoperative member. As some additional illustrative examples, themedically operative member can be: (i) a magnetic resonance (MR)scanner, the array of photosensors being disposed on a bore of the MRscanner; (ii) a positron emission tomography (PET) scanner, the array ofphotosensors being disposed on a bore of the PET scanner; or (iii) aradiation therapy system, the array of photosensors being disposed on orin fixed spatial relation with a radiation emissive member of theradiation therapy system.

With reference to FIG. 6, an example is described in which the medicallyoperative member is a magnetic resonance (MR) scanner 100 including amain magnet 101 housed by a housing 102 to generate a static (B₀)magnetic field in a bore 104 of the housing 102. In some embodiments,the magnet 101 is a superconducting magnet that is disposed in acryogenic shroud 106. In other embodiments, a resistive magnet is usedand the shroud is optionally omitted. A set of magnetic field gradientwindings 108 are disposed on or in the housing 102 to superimposeselected magnetic field gradients on the static (B₀) magnetic field. Oneor more radio frequency coils are provided to excite and detect magneticresonance, such as an illustrated whole body birdcage-type radiofrequency coil 110 mounted on or in the housing 102, and/or one or morelocal radio frequency coils (not shown) disposed in the bore 104 near aregion of interest of the subject 74. The excited and detected magneticresonance signals are suitably spatially encoded by magnetic fieldgradients generated by the gradient coil windings 108 to enablereconstruction of an image from the spatially encoded magnetic resonancesignals.

The region to be imaged is preferably substantially centered in the bore104, at which bore center the static (B₀) magnetic field and appliedmagnetic field gradients are typically most uniform. The portion of thesubject 74 near the center of the bore 104 is typically substantiallyoccluded from view by the magnetic resonance scanner. Thus, camera-basedposition-determining systems are difficult or impossible to use in thissetting.

Accordingly, as shown in FIG. 6, an array of photosensors 170 isdisposed on an inner surface of the generally cylindrical bore 104 so asto view the examination region in the bore 104, and one or moreoptically detectable markers 172, such as LEDs, reflective spheres, orso forth, are disposed with the portion of the subject 74 to be imaged,that is, directly or indirectly on the subject. For example, theoptically detectable markers 172 in some embodiments may be disposed ona local radio frequency coil (not shown) that in turn is disposed on orsecured to the subject 74. The array of photosensors 170 is suitably anamorphous silicon-based photosensor array such as is described, forexample, in Schiebel et al., or an array of CCD devices, or so forth.The illustrated array of photosensors 170 is a generally annular arrayencircling the subject 74, thus advantageously providing 360° viewing.In other embodiments, a hemispherical or otherwise-shaped array ofphotosensors may be used. In embodiments in which the photosensors areamorphous silicon-based and the substrate is a flexible plasticmaterial, the illustrated annular array can be constructed using annularsheets of photosensor arrays. On the other hand, if a rigid substratesuch as glass is used, then the annular array of photosensors 170 can beconstructed of a plurality of adjacent planar segments arranged toapproximate an annular shape. The array of photosensors 170 and the oneor more optically detectable markers 172 are made of non-magneticmaterials such as amorphous silicon, gallium arsenide or other groupIII-arsenide materials, gallium nitride or other group III-nitridematerials, or so forth. Both plastic and glass substrates can be madesuitably non-magnetic.

The position-determining system employing the array of photosensors 170and the one or more optically detectable markers 172 can be operatedsimilarly to the corresponding systems mounted on a gamma cameradescribed with reference to FIGS. 1-4. For example, wide viewing anglephotosensors can be used in conjunction with the Anger-type logicelectronics 82 a analogous to the gamma camera embodiments describedwith reference to FIG. 3, or collimated photosensors can be used inconjunction with backprojection electronics 82 b or anotherprojection-intersection based position locating member analogous to thegamma camera embodiments described with reference to FIG. 4. In thelatter case, the annular shape of the array of photosensors 170advantageously provides a large viewing angle span that facilitatesaccurate position determination for the one or more optically detectablemarkers 172.

Gamma camera and magnetic resonance scanner embodiments are illustratedas examples. As another example, the medically operative member can be aPET scanner with a ring of detectors, and the photosensors can bedisposed on the detector ring, for example as an annular array ofphotosensors. Moreover, while the application of tracking subject motionhas been described as an illustrative application, the positiondetermining systems and methods disclosed herein can be used for otherapplications. For example, if the optically detectable markers are keptin place on the subject as the subject is moved from one imagingmodality to different imaging modality, and both imaging modalities havesuitable photosensors and position-determining members to determine thepositions of the optically detectable markers, then this information canbe used to spatially register images acquired by the two imagingmodalities. For example, if the subject is imaged by both the gammacamera of FIG. 1 and the magnetic resonance scanner of FIG. 6, and theoptically detectable markers 72, 172 are identical, then the markerpositions determined by the position determining systems of the gammacamera and magnetic resonance scanner can be used to spatially registerthe SPECT and magnetic resonance images acquired by the gamma camera andmagnetic resonance scanner, respectively.

The preferred embodiments have been described. Modifications andalterations may occur to others upon reading and understanding thepreceding detailed description. It is intended that the invention beconstrued as including all such modifications and alterations insofar asthey come within the scope of the appended claims or the equivalentsthereof.

1. A position measurement system comprising: an array of photosensorsconfigured to be disposed on a medically operative member configured tointeract with or acquire data from a subject disposed in an examinationregion, the array of photosensors arranged to view the examinationregion; and a position-determining member configured to determine aposition of an optically detectable marker disposed with the subject inthe examination region based on light from the optically detectablemarker sensed by the array of photosensors.
 2. The subject detectionsystem as set forth in claim 1, further comprising: said at least oneoptically detectable marker configured to be disposed with the subjectin the examination region.
 3. The subject detection system as set forthin claim 2, wherein said at least one optically detectable markercomprises a light emitting marker.
 4. The subject detection system asset forth in claim 1, further comprising: a collimator disposed betweenthe array of photosensors and the examination region, theposition-determining member determining a position of a opticallydetectable marker based on light from the optically detectable markersensed by the array of photosensors and defining optical projectiondata.
 5. The subject detection system as set forth in claim 4, whereinthe array of photosensors provide optical projection data for theoptically detectable marker from at least two different angles, and theposition-determining member determines the position of the opticallydetectable marker based on an intersection of optical projections ofdifferent angles.
 6. The subject detection system as set forth in claim1, wherein the photosensors of the array of photosensors have viewingangles effective for a substantially contiguous group of photosensors ofthe array of photosensors to simultaneously sense light from theoptically detectable marker, and the position-determining memberdetermines the position of the optically detectable marker based onrelative light intensities sensed by the photosensors of thesubstantially contiguous group of photosensors that sense light from theoptically detectable marker.
 7. The subject detection system as setforth in claim 1, further including said medically operative member,wherein said medically operative member comprises at least one detectorhead including an array of radiation detectors and a collimator disposedbetween the array of radiation detectors and the examination region, thearray of photosensors being configured to be disposed one of (i) betweenthe collimator and the array of radiation detectors, and (ii) on a faceof the collimator proximate to the examination region.
 8. A medicalsystem comprising: at least one medically operative member configured tointeract with or acquire data from a subject disposed in an examinationregion; an array of photosensors disposed on the at least one medicallyoperative member, the array of photosensors arranged to view theexamination region; and a position-determining member configured todetermine a position of at least one optically detectable markerdisposed with the subject in the examination region based on light fromthe at least one optically detectable marker sensed by the array ofphotosensors.
 9. The medical system as set forth in claim 8, furthercomprising: said at least one optically detectable marker comprising alight-emitting marker configured to be disposed with the subject in theexamination region.
 10. The medical system as set forth in claim 8,wherein the at least one medically operative member comprises: at leastone radiation detector having a radiation-sensitive face configured todetect radiation of interest, the array of photosensors being generallyplanar and arranged parallel with the radiation-sensitive face.
 11. Themedical system as set forth in claim 10, further comprising: acollimator mounted on the radiation-sensitive face of the at least oneradiation detector and disposed between the generally planar array ofphotosensors and the examination region to collimate both radiation ofinterest and light such that radiation of interest detected by theradiation-sensitive face defines projection data and light sensed by thegenerally planar array of photosensors defines optical projection data,the position-determining member comprising electronics configured toreconstruct the optical projection data to determine the position of theat least one optically detectable marker disposed with the subject. 12.The medical system as set forth in claim 11, wherein the at least oneradiation detector comprises a plurality of movable detector heads andthe electronics are configured to reconstruct the combined opticalprojection data from the generally planar arrays of photosensorsdisposed on the radiation-sensitive faces of the plurality of detectorheads.
 13. The medical system as set forth in claim 10, furthercomprising: a collimating microlaminated optical filter disposed betweenthe generally planar array of photosensors and the examination region tocollimate a viewing angle of the photosensors such that light sensed bythe generally planar array of photosensors defines optical projectiondata, the position-determining member comprising electronics configuredto reconstruct the optical projection data to determine the position ofthe at least one optically detectable marker disposed with the subject.14. The medical system as set forth in claim 10, wherein thephotosensors have a viewing angle effective for a substantiallycontiguous group of photosensors of the array of photosensors tosimultaneously sense light from a nearby optically detectable marker,and the position-determining member comprises: electronics configured todetermine the position of the nearby optically detectable marker basedon relative sensed light intensities of the substantially contiguousgroup of photosensors of the array of photosensors.
 15. The medicalsystem as set forth in claim 8, wherein the at least one medicallyoperative member comprises: a medical imaging system including agenerally cylindrical bore surrounding the examination region, the arrayof photosensors being disposed on an inner surface of the generallycylindrical bore so as to view the examination region.
 16. The medicalsystem as set forth in claim 8, wherein the at least one medicallyoperative member is selected from a group consisting of: (i) a detectorhead of a gamma camera, the array of photosensors being disposed on aradiation-sensitive face of the detector head; (ii) a magnetic resonance(MR) scanner, the array of photosensors being disposed on a bore of theMR scanner; (iii) a positron emission tomography (PET) scanner, thearray of photosensors being disposed on a detector ring of the PETscanner; and (iv) a radiation therapy system, the array of photosensorsbeing disposed on or in fixed spatial relation with a radiation emissivemember of the radiation therapy system.
 17. The medical system as setforth in claim 8, wherein the array of photosensors comprises one ormore arrays of amorphous silicon-based photosensors.
 18. The medicalsystem as set forth in claim 8, further comprising: a collimating memberdisposed between the array of photosensors and the examination region tocollimate a viewing angle of the photosensors such that light sensed bythe array of photosensors defines optical projection data, theposition-determining member comprising electronics configured todetermine the position of the at least one optically detectable markerbased on the optical projection data.
 19. The medical system as setforth in claim 8, wherein the position-determining member comprises:electronics configured to determine the position of the at least oneoptically detectable marker based on analysis of light intensitiessensed by a contiguous group of photosensors of the array ofphotosensors.
 20. The medical system as set forth in claim 8, whereinthe at least one medically operative member comprises: a medical imagingsystem configured to collect data from the subject and an imagereconstruction member configured to reconstruct an image of at least aportion of the subject based on the collected data, the imagereconstruction member further configured to adjust at least one of thecollected data and the reconstructed image based on subject positioninferred from the optical marker position of the at least one opticallydetectable marker determined by the position-determining member.
 21. Themedical system as set forth in claim 8, wherein the at least onemedically operative member substantially occludes from view at least arelevant portion of the subject with which the at least one medicallyoperative member is configured to interact or from which the at leastone medically operative member is configured to acquire data.
 22. Themedical system as set forth in claim 8, wherein the at least onemedically operative member is configured to move respective to thesubject during operation of the medically operative member, and thearray of photosensors disposed on the at least one medically operativemember moves in fixed relative spatial relationship with the at leastone medically operative member during operation of the medicallyoperative member.
 23. A position measurement method comprising:detecting at least one optically detectable marker disposed on a subjectin an examination region using photosensors disposed on a medicallyoperative member configured to interact with or acquire data from thesubject in the examination region; and determining at least one opticalmarker position corresponding to the at least one optically detectablemarker based on the detecting.
 24. The position measurement method asset forth in claim 23, further comprising: adjusting at least one of (i)data acquired from the subject and (ii) an image reconstructed from dataacquired from the subject based on subject position inferred from the atleast one optical marker position.
 25. The position measurement methodas set forth in claim 23, further comprising: repeating the detectingand determining a plurality of times to track motion of the at least oneoptical marker position over time.